I. Field of the Invention
The present invention relates generally to a hydraulic rotary actuator of the vane type, and more particularly to such a rotary actuator having improved seal structures and a fail-safe brake mechanism.
II. Discussion of the Prior Art
Hydraulic rotary actuators find application in a number of fields where it is desired to rotate a load through a predetermined angle relative to a stationary frame member. For example, a few applications use rotary actuators for controlling the positioning of a bucket on the end of a boom on a power company's utility truck, the rotation of the bucket on a backhoe and tipping refuse containers on refuse collection vehicles.
A typical vane-type hydraulic rotary actuator comprises a stator housing with a rotor journaled for rotation within the housing that defines an annular chamber between the rotor surface and the stator. Affixed to the rotor and cooperating with a cylindrical wall of the stator is a vane that divides the chamber into a pair of hydraulic fluid receiving chambers. Also disposed in the chamber and fixedly attached to the stator is a stop having opposed ends separated by a predetermined arc. Injection of high pressure hydraulic fluid into one of the pair of chambers forces the vane and, therefore, the rotor to shift through a predetermined angle until the vane strikes a first end of the stop. The movement of the vane also forces hydraulic fluid, now at a low pressure, out from the other of the two chambers. By reversing the chamber to which the high pressure hydraulic fluid is applied, the vane will now move in the opposite direction until reaching the other end of the stop. A typical prior art example of a vane type rotary actuator is disclosed in the French U.S. Pat. No. 3,198,090.
Also known in the art is the need for effective seals for preventing leakage of the high pressure hydraulic fluid past the vane which seriously detracts the efficiency of the hydraulic actuator and results in rotor drift. The sealing arrangements to date have an inherent leak path near the base of the vane where the shaft protrudes from the actuator. It is very difficult to prevent leakage across the vane and along the shaft (out of the actuator unit).
Furthermore, a need exists for an effective brake arrangement that will lock the rotor relative to the stator in the event of a loss of pressure, such as may result from a leak in a hydraulic line or the intended stoppage of the rotor with the vane part way between the opposed ends of the stop. Because seal leakage can result in unwarranted drifting of the load, it is known in the art to provide a brake arrangement or the incorporation of counterbalance valves. U.S. Pat. No. 3,179,018 to Rumsey describes a hydraulic rotary actuator that incorporates a hydraulic braking system for locking the rotor in any position throughout the operating range of the rotary actuator. By applying hydraulic fluid under pressure to a piston secured to the rotor, it is brought into contact with a brake shoe forming a part of the stator housing. The implementation disclosed in the '018 patent requires the application of hydraulic pressure to the brake assembly at all times, except when it is desired to reposition a load using the hydraulic actuator. Should the hydraulic brake line fail, the load would be free to swing which, of course, is highly undesirable and unsafe.
A need, therefore, exists for a vane-type hydraulic actuator having an internal brake that is fail safe, i.e., the brake force is applied to lock the rotor in the event that hydraulic pressure releasing the brake falls.